Temperature or velocity measurements routinely are taken to provide data for responsive action. Although a wide variety of sensors have evolved, a particular type of sensor that has found widespread application is the thermistor. A thermistor is a special resistor that varies in resistance with temperature. Electric current is passed through a thermistor to generate a voltage across the thermistor. This voltage can be measured and calibrated to represent temperature. However, the current through any resistor also produces heat and this heat raises the temperature of the thermistor to cause an error in temperature measurement.
When thermistors are used to measure temperature profiles in the sea, for example, a thermistor is moved slowly up and down through the water from a floating platform or a chain of thermistors are suspended from the platform. In temperature sensing applications the electrical current through the thermistors must be low to minimize self heating errors.
Limitations to these applications of thermistors arise since the resolution of the temperature measurement is limited by the accuracy with which the voltage generated across the thermistor by the low current can be measured. Another limitation on the accuracy is that the signal processing equipment is located in a different environment than the thermistor so that the consequent temperature variations in the equipment produce errors that obscure small variations from the thermistor. A third limitation on the accuracy is the errors attributed to the different resistances of the different lengths of wire connecting the thermistors in a chain to the remote signal processing equipment. A fourth limitation on the accuracy is the negative characteristic of resistance of the thermistors with temperature which reduces resolution at higher temperatures.(A positive characteristic of resistance of the thermistor with temperature would reduce the accuracy at lower temperatures). The difference in resolution is caused by the exponential change in resistance of the thermistor with temperature.
Thus, there is a continuing need in the state of the art for an apparatus and method to improve and simplify the measurement of temperature by using thermistors to control the current output of electrical current sources.